The present invention relates to a power transmitting device.
The power transmitting device disclosed in Japanese Laid-Open Patent Publication No. 2004-108575, for example, is provided with a cylindrical rotating member which rotates when a driving force is inputted and a rotary shaft member which is placed within the cylindrical rotating member so as to be freely rotatable and coaxial with the cylindrical rotating member. In the case of the device disclosed in this document, a clutch mechanism provided between the cylindrical rotating member and the rotary shaft member permits torque to be transmitted between the cylindrical rotating member and the rotary shaft member. This power transmitting device is provided with a cam mechanism as shown in FIG. 7. The cam mechanism is provided with spherical bodies 83 and a stopper body 84 between a main cam 81 and a pilot cam 82. In this cam mechanism, the main cam 81 and the stopper body 84 are urged by a torsion spring 85 to be rotated so that a protrusion 86 of the main cam 81 and an engaging protrusion 87 of the stopper body 84 engage with each other.
The pilot cam 82 receives the rotation of the engine provided in the front portion of the vehicle through an electromagnetic clutch and a pilot clutch. When the rotational torque generated in the pilot cam 82 exceeds the urging force of the torsion spring 85, the pilot cam 82 and the main cam 81 rotate relative to each other. Thus, as the result of this relative rotation, the spherical bodies 83 between the two cams 81 and 82 move the main cam 81 in the axial direction. Then, the driving force is transmitted from the engine to the rear wheels through a main friction clutch (not shown). On the other hand, when the vehicle is being towed, even if the electromagnetic clutch is turned off so that a drag torque resulting from the rotation of the rear wheels is generated in the pilot clutch, the relative rotation of the pilot cam 82 and the main cam 81 is restricted in the case where the torque is no greater than the urging force of the torsion spring 85. Therefore, the main friction clutch is prevented from being erroneously activated.
In the above described power transmitting device, clutch plates of the main friction clutch sometimes rub against each other even when the main friction clutch is not engaged. In addition, a great number of main frictional clutches are a wet type clutch, in which lubricant exists between the respective clutch plates is formed. Therefore, in the case where the number of rotations of the front wheels becomes greater than that of the rear wheels, for example, when a spare tire for emergencies is used or at the time of quick acceleration, a problem arises where part of the torque becomes drag torque due to the viscosity of the lubricant intervened between the clutch plates, and is transmitted to the driven side.